1. Field of the Invention
This invention relates to a method for reducing the amount of beverage soluble metal ions in a diatomaceous earth filter aid, a method for filtering a beverage using the treated diatomaceous earth filter aid, and a method for increasing the flavor stability of a malt beverage filtered with a diatomaceous earth filter aid.
2. Description of the Related Art
Filtration using diatomaceous earth as a filtration aid has been used by brewers around the world for many years to produce brilliantly clear beer. Diatomaceous earth is a very light weight powder with high porosity and its chemical composition consists of 86% silicon, 5% sodium, 3% magnesium, 2% iron, and many other minor metals. The porous structure of diatomaceous earth is particularly effective in entrapping suspended particles. Diatomaceous earth can also be called kieselguhr.
In a common beer filtering process, a filter element is pre-coated with a mixture of diatomaceous earth and diluent (e.g., deaerated water) prior to the filtration. Another portion of the diatomaceous earth mixture (body feed) is added directly into the beer stream being filtered. It usually requires two filtrations (primary and final) to achieve beer clarity.
The “cardboard” like flavor that occurs in stale beer is thought to arise from free radical mediated oxidation of various constituents in beer. The characteristic odor and taste are believed to be caused by decomposition products from the free radical process. It is reported that these off-flavor products can be detected by the consumer in beer even at very low concentrations. Diatomaceous earth filtration can be a major liability for beer flavor stability because some of the iron, copper and other metals from the diatomaceous earth leach into the beer being filtered such that trace amounts of iron or copper and trace levels of oxygen mediate free radical formation resulting in decreased flavor stability of the beer.
Because iron (typically as the ions Fe2+ and/or Fe3+) can decrease flavor stability of the beer, the contents of iron in the diatomaceous earth which are soluble in beer (e.g., beer soluble iron) provide useful indicators of the degree to which release of iron occurs. It has been reported that the majority of conventional diatomite filter aids used in beer filtration have beer soluble iron contents from approximately 7 mg Fe/kg product to approximately 50 mg Fe/kg product. A reliable analytical method has been established in the industry to determine the solubility of iron from diatomite products in beer (American Society of Brewing Chemists, 1987).
Accordingly, processes have been developed which seek to reduce the beverage soluble iron content of diatomaceous earth filter aids. For example, U.S. Pat. No. 665,652 describes heating diatomaceous earth in a solution of hydrochloric acid and nitric acid. U.S. Pat. No. 1,992,547 describes contacting diatomaceous earth with hydrochloric acid, sulfuric acid, or phosphoric acid. U.S. Pat. No. 2,701,240 describes contacting diatomaceous earth with oxy-acids of phosphorous. U.S. Pat. No. 4,134,857 describes contacting diatomaceous earth with tannic acid or gallic acid. U.S. Pat. Nos. 4,142,968 and 4,187,174 disclose a method in which diatomaceous earth filter aids are treated with sulfuric acid, hydrochloric acid, nitric acid, acetic acid, or oxalic acid. In this method, the filter aids are maintained in contact with a relatively small volume of an acid for a period of days, resulting in a beer soluble iron content of less than 100-50 ppm. U.S. Patent Application Publication No. 2001/0023233 discloses the washing of diatomite products with a mineral acid (such as sulfuric acid, hydrochloric acid, phosphoric acid, or nitric acid), or an organic acid (such as citric acid or acetic acid). However, in a full-scale brewery, the preparation of acid-washed diatomaceous earth creates extra work and waste water, including washing diatomaceous earth, filtering and disposing of the acidic wash solution, and re-suspending the diatomaceous earth.
U.S. Pat. No. 4,965,084 describes another method in which the beer soluble iron content of a diatomaceous earth material is reduced by contacting the diatomaceous earth with a polydentate ligand. U.S. Pat. No. 5,099,906 describes the treatment of diatomaceous earth filter aids with an alkali metal silicate to reduce multivalent metal cations. U.S. Patent Application Publication No. 2009/0264287 (also WO 2008/024952) discloses an alternative method in which the beer soluble iron content of a diatomaceous earth material is reduced by subjecting the diatomaceous earth feed material to a saturated steam treatment. These methods require a pre-cleaning procedure including separation of soluble iron or other metal ions from diatomaceous earth prior to use as a filter aid, resulting in additional operational and equipment costs.
Brewers and brewing scientists around the world are looking at alternative technologies to replace diatomaceous earth filtration. One proposed method for filtering beer uses a polymer for heavy metal reduction. For example, Divergan® HM, a crosslinked copolymer consisting of vinylimidazole and vinylpyrrolidone available from BASF Corporation, Mount Olive, N.J., USA, has been developed for heavy metal reduction. Membranes have also been considered for beer filtration. One stumbling block is the cost of these alternative filtration technologies.
Therefore, there still exists a need for an improved method for preventing soluble iron, soluble copper, and other soluble cations in diatomaceous earth filter aids from being transferred into filtered beer.